Hey Nick,
FF 56 is definetly capable of generating the waveform & playing back the audio during upload (since the file is uploaded in the background, the local file is used for playback during the upload/chord entering phase).
The only reasons I could think of why this could fails are:
either, you have very little memory available in your device (browsing with a smartphone/tablet?),
some security add-on may be blocking that functionality (are you using ad-block or such?) or
You may have a broken javascript file stuck in your browser cache, please press CTRL+F5 when visiting the initial page of the upload process to make sure all external Javascript files are reloaded from the server.

If none of that seems to lead at curing the issue, feel free to get back to me and we can check if there are any notable error messages showing up in the browser console.
If you cannot generate the waveform yourself, it will be auto-generated some time after your upload, and you may still add & edit the color bar markup via the "edit track" page after the track has been published, so neither issue is without a workaround.
All the best
Richard